Titanium dioxide is commonly produced as a white pigment in a finely divided substantially spheroidal form and in either the anatase or rutile modification, and as such is used extensively as a pigment in paint systems, ceramics, plastics and the like. Titanium dioxide has also been produced in fibrous or acicular form in which form it is used extensively as thermal insulating material, and as a reinforcing component in ceramics, cermets, plastic and paper. An acicular or fibrous TiO.sub.2 is described in U.S. Pat. No. 3,012,857 wherein TiO.sub.2 in the form of fibers having a cross-section of about 1 micron and length of from 0.5 to 1.0 mm were prepared by contacting oxygen with a molten alkalinous metal halide containing a titanium subhalide dissolved therein. This fibrous TiO.sub.2 was developed for use as a reinforcing component in paper, plastics and the like. U.S. Pat. No. 3,030,183 also describes production of fibrous TiO.sub.2 - in this instance the TiO.sub.2 being of the rutile modification and formed by adding titanium tetrachloride and oxygen to an alkali metal or alkaline earth metal halide melt. The use of molten salts to produce fibrous TiO.sub.2 is further illustrated by U.S. Pat. Nos. 3,241,922; 3,244,481; 3,338,677 and 3,650,693.
A second approach to the preparation of acicular or fibrous TiO.sub.2 is disclosed in U.S. Pat. No. 3,331,660 wherein acicular TiO.sub.2 having predominantly either the anatase or rutile crystal modification is prepared by calcining alkali metal deficient titanate fibers at temperatures from 800.degree.-950.degree. C. - the titanate fibers being formed by treating potassium, sodium or other alkali metal titanates with acidic agents, i.e. mineral acids, TiCl.sub.4 or chlorine gas. In this connection, the prior art is replete with disclosures relative to processes for making fibrous alkali metal titanates as illustrated by U.S. Pat. Nos. 2,841,470; 3,328,117 and 3,380,847.
Still another approach to making fibrous TiO.sub.2 of the rutile or anatase crystal modification is illustrated by U.S. Pat. Nos. 3,329,484 and 3,519,310, each of which describes production of a monocrystalline acicular rutile TiO.sub.2 - the former patent by mixing a colloidal suspension of TiO.sub.2 nucleating seed particles with an acidic titanium chloride solution and heating the mixture at temperature of at least 190.degree. C., under pressure, to hydrolyze the titanium as acicular TiO.sub.2 ; while the process of the latter patent comprises calcining, in nonflowing solid state, a blended mixture of (a) a preformed TiO.sub.2 comprising a seeding amount of rutile TiO.sub.2, (b) a salt such as sodium chloride with or without another alkali metal salt and (c) an oxyphosphorous compound - calcination being carried out at temperatures from 725.degree.-1000.degree. C. for from 1/2 to 10 hours. The calcined product is leached to remove soluble salts.